Neurotransmitter receptors and ion channels are multimeric protein complexes that show tremendous structural and functional diversity. Their properties are defined not only by the integral membrane proteins that constitute the essential ligand binding or pore-forming domains, but also by the other transmembrane or cytoplasmic proteins that associate with this core to confer specific activation, modulation, or subcellular distribution characteristics on the native receptor complex. To appreciate fully the function of a receptor or channel, we must understand how structurally and functionally diverse proteins interact to form the unit that constitutes the physiologically relevant signaling complex in vivo. Extracellular ATP and other nucleotides serve as neurotransmitters by activating G protein-coupled receptors and ligand-gated ion channels (P2Y and P2X receptors, respectively). Little is currently known about the structural configuration of these receptor complexes and whether they interact with other cellular proteins to achieve proper membrane targeting or signaling characteristics. Experiments described in this proposal are aimed at determining whether P2Y receptors function as homo- or heterodimeric complexes (as observed for some other metabotropic transmitter receptors). If so, then the functional consequences of such interactions will be assessed using a variety of pharmacological, electrophysiological, and histological methods. Another aim is to determine whether P2X receptors interact with cytoskeletal proteins to regulate channel distribution or function, most notably, desensitization rates and ion selectivity. These questions will be approached using genetic and biochemical methods to detect and characterize specific protein-protein interactions, as well as electrophysiological and histological methods to understand the physiological significance of such interactions. Finally, the concept of receptor multivalency will be exploited to search for ligands having increased potency or selectivity at specific P2 receptor complexes.